Geometric Design Practices for Resurfacing, Restoration, and Rehabilitation (2011) / Chapter Skim
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Pages 22-32
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From page 22... ...
For many speed-related elements, Chapter 11 presents an acceptable threshold value for the element which is considerably below the 3R design speed. For example, if the calculated design speed of an existing crest vertical curve is within 15 mph of the 3R project design speed and there is not an adverse accident history, the existing crest vertical curve may be retained in the project design without a design exception.
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From page 23... ...
of States Lane Width 31 Shoulder Width 31 Bridge Width 28 Horizontal Alignment 28 Vertical Alignment 28 Design Speed 28 Cross Slope 26 Superelevation 24 Structural Capacity 24 Stopping Sight Distance 23 Horizontal Clearance 23 Vertical Clearance 20 Grade 20 Although most states discuss each of these design elements to varying degrees, a few simply present the guidelines in a table without further explanation. For example, Table 10, extracted from Michigan's road design manual, shows 3R minimum guidelines for the 13 controlling criteria applicable to non-NHS roads.
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Hence, it is critical for 3R projects to set design criteria for these two elements. Table 12 shows the minimum lane and shoulder widths for two-lane rural highways recommended in TRB SR 214.
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In TRB SR 214, there are two columns labeled "Combined Lane and Shoulder Width." In preparing Table 12, it is assumed that the shoulder width is the combined lane and shoulder width minus the lane width. TABLE 11 3R DESIGN SPEED VERSUS POSTED SPEED (Florida DOT)
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Structural Capacity This design element refers to bridge structural capacity. Bridges are usually designed to accommodate either an H-15 or HS-20 loading.
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Horizontal alignment influences stopping sight distance, another primary controlling criterion. TRB SR 214 has two recommendations regarding horizontal curvature (and superelevation)
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As horizontal curves are to horizontal alignment, vertical curves are to vertical alignment. Vertical curvature influences stopping sight distance, another primary controlling criterion.
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Because stopping sight distance is integral to horizontal and vertical alignment, many states discuss it within these two alignment features. For example, Wisconsin has the following guideline for crest vertical curves that is based on stopping sight distance: All crest vertical curves with an existing design speed based on stopping sight distance provided, not within 15 mph of the overall project design speed shall be upgraded on highways with a design traffic volume over 1,500 AADT.
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TABLE 17 STOPPING SIGHT DISTANCE REQUIRED FOR 3R PROJECTS IN FLORIDA Design Speed (mph) Stopping Sight Distance (ft)
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Only a few states discuss vertical clearance in their 3R policy. Those that do cite 14 ft as a minimum for keeping a structure as is, and as low as 13.5 ft for non-NHS routes.
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As with most states, Florida's guidelines call for meeting ADA requirements on detectable warnings and curb ramps. For bicyclist needs, Florida has guidance for providing space within the travel way for bicycles; bicycle lanes at right-turn lanes; and proper treatment of drainage inlets, grates, and utility covers.
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